A lunar occultation of the dust-scattering halo around GX 5-1 observed with ROSAT

The x-ray source GX5-1 in he galactic bulge has been observed with the position-sensitive proportional counter onboard the Rontgen satellite (ROSAT) during and after a lunar occultation. Extended emission around the source was unambiguously discovered while the central source was behind the lunar rim. This emission is interpreted as a dust-scattering halo around GX 5-1 that has a fractional intensity of 28 percent, implying a grain column density between GX 5-1 and Earth of 3 X [10.sup.10] per square centimeter. The halo derived from imaging during the ROSAT all-sky survey is identical to that obtained from the lunar occultation, thus demonstrating that the ROSAT x-ray mirror scattering has not changed as compared with the mirror properties as measured in preflight calibrations.
In the 1960s, the possible existence of halos, produces by the scattering of x-rays off dust grains in the interstellar medium, around the stronger and moderately absorbed x-ray sources was [...]